PROJECT SUMMARY/ABSTRACT The rising number of individuals with legal access to marijuana for medical and/or recreational use is arguably the most profound change in drug policy in the past 20 years given the potential impact on public health. California, one of 28 states that authorized marijuana for medical use since 1996, became the eighth medical marijuana state to legalize marijuana for recreational use following the passage of the Adult Use of Marijuana Act (AUMA) in 2016. No research has studied the real-time transition from a medical to a recreational environment and its impact on young adult marijuana users since states began legalizing marijuana for recreational use in 2012. Furthermore, no study has examined the impact of legalization for recreational use on 18 to 20 year olds ? a group eligible for medical use but prohibited from recreational use in all legal states. Our long-term goal in this renewal of a current R01 is to inform public policy with findings on safer and controlled use of marijuana among young adults in an environment where marijuana is legal for recreational use. Our objective here is to determine the impact of legalization on young adult marijuana users (18 to 30 year olds) by continuing the study of a cohort of young adults ? 210 medical marijuana patients (MMP), 156 non-patient marijuana users (NPU) - recruited in Los Angeles, CA in 2014-15 while also enrolling a new cohort of 150 18 to 20 year old marijuana users. Our central hypothesis is that legalizing marijuana for recreational use will result in increased marijuana use, other drug use, and risk behaviors among young adults, but this increase will vary by patient status (MMP vs. NPU) and orientation towards marijuana use, i.e., medicinal vs. recreational. Our rationale for the proposed research is that renewing a current R01 will offer the unique opportunity to: analyze marijuana use, other drug use, and risk behaviors on a cohort of young adult marijuana users continuously collected both before and after legalization in California; and recruit a new underage cohort of marijuana users for comparative purposes. Guided by preliminary data from the current R01, the central hypothesis will be tested by pursuing three specific aims: 1) Determine the impact of marijuana legalization on trajectories of marijuana use, including high-potency forms, e.g., dabs, edibles, among MMP and NPU (aged 21 to 30); 2) Determine the impact of marijuana legalization on patterns of other drug use and risk behaviors among medicinal and recreational users (aged 21 to 30); and 3) Determine the impact of marijuana legalization on attitudes and patterns of marijuana use among a newly recruited cohort of underage young adults (aged 18 to 20). The approach is innovative because it uses an interrupted time series design to study the effects of legalizing marijuana on young adults, includes both legal and non-legal recreational users, and includes medicinal orientation of marijuana use among recreational users in a fully legal context. The proposed research is significant since the public health impact of legalizing marijuana for recreational use among young adults is unknown.